Salmonella spp. are facultative intracellular pathogens causing localized or systemic infections, in addition to a chronic asymptomatic carrier state. They are of worldwide economic and public health significance. In poultry, fowl typhoid and pullorum disease continue to cause economic losses in those parts of the world where the poultry industries are continuing to intensify and where open sided housing is common. A number of serotypes that cause human gastroenteritis are also increasing. The costs or impracticality of improvements in hygiene and management together with increasing problems of antibiotic resistance indicates that vaccination in poultry will become more attractive as an adjunct to existing control measures (Zhang-Barber L. et al., Vaccine, 1999, 17(20-21): 2538-45).
Salmonella is one of the major causes of food-borne illnesses in humans. According to the Commission's report on zoonoses (European Commission: Trends and sources of zoonotic infections in animals, feed, food and man in the European Union and Norway in 2003), 135,546 human cases of salmonellosis have been reported in 2003 by the 15 Member States of the European Union and Norway.
The poultry industry, especially in Europe and in the US, is under strong pressure from health authorities and consumers to reduce the risks of human contamination with Salmonella of poultry origin, particularly salmonellosis (pathogen reduction and HACCP in the US, Council Directive 92/117/EEC in the EU).
As Salmonella infects many animal populations (e.g., mammalian, avian), the risk of suffering salmonellosis always exists, whatever the country, the season or the food handling practices.
The zoonotic Salmonella spp. causes of human gastrointestinal infection have been treated with existing antimicrobials. Since the early 1990s, strains of Salmonella resistant to a range of antimicrobials have appeared, making the treatment of infection less efficient and increasing the human risk of contracting a gastrointestinal infection caused by Salmonella spp.
As in previous years, Salmonella enterica subspecies enterica serovar Enteritidis (Salmonella Enteritidis) dominated, causing 61.8% (2002: 67.1%) of all notified cases in the European Union and Norway. Rates in the individual countries ranged between 87.9% in Austria and 33.3% in France. Salmonella enterica subspecies enterica serovar Typhimurium (Salmonella Typhimurium) was second, causing 16.5% of all cases. Rates in the individual countries ranged between 5.8% in Austria and 28.7% in Ireland. As in previous years, after Salmonella Enteritidis and Salmonella Typhimurium, most cases were caused by Salmonella enterica subspecies enterica serovar Virchow (Salmonella Virchow), Salmonella enterica subspecies enterica serovar Infantis (Salmonella Infantis) and Salmonella enterica subspecies enterica serovar Hadar (Salmonella Hadar). Each of these serovars is involved in less than 1% of all notified cases.
The major source of contamination is through egg and poultry meat consumption. The reduction of such risks is achieved through a combination of means all along the egg and meat production chains, such as good farming, hygienic practices and vaccination.
Community legislation on food hygiene and control of zoonoses includes a number of provisions that seek to control and prevent the Salmonella contamination of foodstuffs. Measures to reduce Salmonella prevalence in live animals are believed to be one of the most effective ways of reducing the contamination of foodstuffs and the number of human salmonellosis cases.
In 2003, the new European legislation on zoonoses was published; Regulation 2160/2003 provides for the setting of pathogen reduction targets along the food chain, mainly for animal populations, and the establishment of national control plans in order to meet these targets. Salmonella spp. is the primary target, in particular the serotypes considered to have public health significance. Targets will be set progressively in different animal populations: breeding flocks of Gallus gallus, laying hens, broilers, turkeys and slaughter pigs. Up to now, targets have been established for breeding flocks of Gallus gallus only (Regulation 1003/2005); the target has been set at 1%, meaning that for the end of 2009 the maximum percentage of flocks positive for Salmonella Enteritidis, Salmonella Typhimurium, Salmonella Infantis, Salmonella Hadar and Salmonella Virchow at the EU level will have to be 1%.
As far as breeders are concerned, Regulation 1091/2005 forbids the use of antimicrobials as control measure against Salmonella, whereas the use of vaccines is accepted and recommended. Conclusions and recommendations of the Scientific Panel on Biological Hazards on a request from the Commission related to the use of vaccines for the control of Salmonella in poultry (The EFSA Journal (2004) 114, 1-74) are largely favourable toward the use of vaccines to control Salmonella at the farm level. In particular, the panel's conclusions are, among others, that:                The basis for successful control of Salmonella infections in poultry farms are good farming and hygienic practices (including all the aspects covering feed, birds, management, cleaning and disinfection, control of rodents etc.) as well as testing and removal of positive flocks from production. Vaccination of chickens is regarded as a measure to increase the resistance of birds against Salmonella exposure and decrease the shedding.        There is experimental and some field evidence that a reduced level of fecal excretion and systemic invasion of Salmonella organisms in vaccinated birds will result in a reduced contamination of table eggs and the environment.        If a control programme is targeting in breeders of layers/broilers or laying hens and the flock prevalence is high, vaccination may be useful in reducing shedding and egg contamination. If the flock prevalence is low, vaccination may not be so useful but still could be used as one of the preventive measures to maintain a low prevalence.        
There are more than 2,000 serovars of Salmonella bacteria.
The Kauffman and White classification permits serological varieties of the genus Salmonella to be differentiated from each other. This scheme differentiates isolates by determining which surface antigens are produced by the bacterium. First, the “O” antigen type is determined. “O” antigens are the polysaccharides associated with the lipopolysaccharide of the bacterial outer membrane. Once the “O” antigen group is determined, the “H” antigen is determined. The “H” antigens are proteins associated with the bacterial flagella. Salmonellas exist in two phases; a motile phase and a non-motile phase. These are also referred to as the specific and non-specific phases. Different “H” antigens are produced depending on the phase in which the Salmonella is found. Pathogenic strains of Salmonella typhi carry an additional antigen, “Vi”, so-called because of the enhanced virulence of strains that produce this antigen, which is associated with a bacterial capsule.
Following the Kauffman and White classification, “O”-groups of Salmonella serovars are formed.
Against colonization by group C Salmonella in chickens, an attenuated vaccine was developed, based on deleted cya/deleted crp Salmonella Hadar and a deleted phoP Salmonella Hadar (Roland K. et al., Avian Dis., 2004, 48(3): 445-52). Although the deleted cya/deleted crp derivative induced higher levels of serum antibody, it did not provide an immune response protective against colonization by Salmonella Hadar.
Chacana et al. (Chacana P. A. et al., Avian Dis., 2006, 50(2): 280-3) demonstrated that Salmonella attenuated vaccine can elicit cross-immunity against members of the same Kauffmann-White scheme serogroup. The protection conferred by TAD Salmonella vac E, an attenuated Salmonella Enteritidis vaccine, was explored against fowl typhoid. Three groups of laying hens were vaccinated with different vaccination schedules starting on the first day of life, and afterwards were infected with 2×105 CFU of a virulent Salmonella Gallinarum strain, either at week 28 or week 52. Mortality, fecal shedding, and organ invasion of Salmonella Gallinarum were assessed. Salmonella Enteritidis vaccine was able to cross-immunize against Salmonella Gallinarum, both strains of the D group Salmonella, according to Kauffmann-White classification. At week 28, hens vaccinated with three oral doses or with two oral doses combined with one subcutaneous dose were protected by the vaccine. However, at week 52, when hens were infected 36 weeks after the final immunization, the vaccine was not able to confer protection.
Due to the great number of Salmonella serovars, there is a need for vaccines against Salmonella that are able to induce a protective immune response against more than one Salmonella serovar and/or against Salmonella of more than one Kauffmann-White classification group.